1. Field of the Invention
The present invention relates to a multicolour inked ribbon cartridge and to a positioning mechanism of such cartridge in an impact serial printer.
2. Description of the Prior Art
An impact serial printer generally consists of a platen, a carriage moving parallel to the platen and a printing head (for example, of the needle type) mounted on the carriage and sequentially actuated during the carriage movement to generate a printed character row on a printing support arranged against the platen. Characters are printed utilizing an inked ribbon, typically black, arranged between the support and the printing head. Because of a need for multicolour printing, the impact printer manufacturers turned to inexpensive mechanisms which, when mounted on a conventional solid colour printer frame, changed it into a multicolour printer. This change was essentially enabled by using an inked ribbon cartridge where the ribbon is comprised of a plurality of longitudinal contiguous bands of different colour, and by providing the printer with an additional mechanism able to position such cartridge in order that the desired ribbon band is positioned between the printing head and the printing support. Most of the multicolour inked ribbon cartridges have a structure generally similar, except for width, to the ones having a solid color inked ribbon cartridges, and, like these, are supported by the printing carriage. In the multicolour printer, the carriage is provided with a cartridge housing platform, which can cover a plurality of positions with respect to the printing head. This plurality of positions is equal to the number of the multicolour ribbon bands. In this way the positioning of a selected band of the multicolour ribbon, between the platen and the printing head is obtained by suitably tilting the platform (and therefore the whole cartridge) relative to the printing head. This tilting can be performed at the end/beginning of a row serial printing process or during the printing process itself. In the first case the platform is provided with mechanisms of a mechanical type (spring, articulated arms, pawls) which interact with suitable cams arranged at the end/beginning of the printing run, as disclosed for example by U.S. Pat. Nos. 4,280,767 and 4,368,993. Solutions of this type greatly limit the printing speed as they compel the printing head to perform one full pass for each required colour, in order to print a character row.
To overcome the above difficulties electromechanical actuators have been utilized to control the mechanical devices enabling the relative shift of the housing platform of the multicolour inked ribbon cartridge relative to the printing head. In this manner several multicolour inked ribbon bands can be selectively interposed between the printing head and printing support during execution of the serial printing process. The electromechanical actuators can be mounted directly on the printing carriage, as disclosed in U.S. Pat. No. 4,407,595 for example. Alternately, in order not to make the carriage heavy and therefore increase its inertia, such mechanical actuators can be suitably attached to the printer frame, in a manner disclosed in U.S. Pat. No. 4,391,540.
A majority of positioning mechanisms of the multicolour inked band contained in a cartridge mounted on the printing carriage have a large number of mechanical components which, besides being expensive, do not allow for their easy assembly and their subsequent mounting on a solid colour serial printing frame. Moreover, mounting of the multicolour inked ribbon cartridges on a carriage according to the prior art is affected by a basic limitation. Because of the small size of the cartridge, the multicolour inked ribbon contained therein is not large enough to provide an adequate useful life of such cartridge. With black ribbon cartridges this problem is not particularly severe because the excellent chemical-physical characteristics of such ink assure that these cartridges can provide for the printing of several millions of printed characters. The life of a ribbon having coloured ink, different than the black one, for instance yellow, is however much lower because of the different chemical-physical characteristics of the ink. A multicolour serial printer therefore needs a multicolour inked ribbon larger than the one normally used for a solid-colour printer and therefore requires a corresponding larger cartridge. This need, together with the need for reducing the number of all the mechanical parts for positioning the multicolour ribbon has recently induced some manufacturers of serial printers to use multicolour inked ribbon cartridges of a larger size which are housed on a suitable platform mounted on the printer frame. These platforms, like the ones mounted on the printing carriage, are able to perform a shift relative as to the printing head. However this solution does not substantially reduce the number of mechanical components comprising the positioning mechanism of the multicolor ribbon. Consequently it is difficult to manufacture such components for use on the frame of a solid-colour serial printer.
Unlike the cartridges mounted on the printing carriage and moving transversely with it, where the ribbon portion between the printing head and printing support is minimum, the use of tranversally fixed cartridges causes the ribbon to be interposed between the printing head and the printing support for the entire length of the platen. As a result the platform itself must be provided with additional tensioning and guiding means so that the interposed ribbon portion does not vibrate between the printing head and the printing support during the printing process, and assures that such ribbon portion follow the movements of the cartridge housing platform correctly. More particularly, it is essential that the inked ribbon is correctly positioned each time in relation to the variable position along the printing line where the printing head is placed. These additional means make the positioning mechanism of the multicolour ribbon more complicated. What was needed was a cartridge that simplifies both the structure and the assembly of the devices which can be mounted on a conventional type printer and changing it into a multicolour printer.